The present invention relates to a stamper protecting layer set in a molding apparatus for use in molding an optical disk, an optical disk molding apparatus with the stamper protecting layer, and an optical disk molding method carried out with the optical disk molding apparatus.
Conventionally, optical disks are molded by a molding apparatus 1 shown in FIG. 6. The molding apparatus 1 is in a concentric structure with respect to a center axis 4, and therefore a left part of the apparatus to the left of the center axis 4 is not illustrated in FIG. 6. The molding apparatus 1 has two molds 2 and 10 made of steel materials, for example a stainless steel. The mold 2 is movable for opening and includes a stamper 3 for forming pits representing recording information on an optical disk to be molded. The mold 10 is fixed and has a sprue 6 to inject a resin therethrough to form the optical disk.
The optical disk is molded with the use of the above molding apparatus 1 in a manner as will be described hereinbelow. After the stamper 3 is set to the mold 2, the resin is injected through the sprue 6 to a cavity 5 defined between the closed molds 2 and 10 where the optical disk is molded. Concaves and convexes 9 formed on the stamper 3 are transferred to the resin. After the transfer of the concaves and convexes 9, the filled resin is cooled and then the mold 2 is opened. When the mold 2 is completely opened, both the resin present in the sprue 6 and a molded body formed by the cavity 5 which becomes the optical disk are pressed up by an ejection rod 7, to thereby separate the molded body from the mold 2. After the ejection, the molded body is carried outside the molding apparatus 1 by a take-out device.
In a sequence of the above processes, when the resin supplied through the sprue 6 is filled into the cavity 5, a tensile force acting in a radial direction of the stamper 3 because of a viscosity of the resin and a thermal stress due to temperature changes during the repeated heating and cooling are exerted to the stamper 3. Meanwhile, the tensile force due to the viscosity of the resin and the thermal stress do not act on the mold 2. As a result of this, a friction force is brought about between the mold 2 and stamper 3 in a direction I which is a diametrical direction of the optical disk, thereby wearing the stamper 3 and mold 2 at a contact part 8. The wear is generally called as an adhesive wear, which involves digging into the mold 2 and stamper 3. Namely, microscopically, contact surfaces of the stamper 3 and mold 2 are not completely flat and have irregularities. Thus, projections in the irregularities cause the formation of adhesive portions and then the adhesive portions formed between the stamper 3 and mold 2 cause digging out from (i.e. next of) the stamper 3 or mold 2 due to the friction force in the I direction. Because of projections and recesses formed consequent to the wear, as shown in FIG. 7, several-.mu.m projecting parts 21 are formed at the contact part 8 of the mold 2 and thus projecting parts 22 corresponding to the projecting parts 21 appear at a recording face 9a of the stamper 3 having the above-mentioned concaves and convexes 9. The projecting parts 22 at the recording face 9a are transferred to the molded body. Therefore, if the projecting parts 22 at the recording face 9a exceed a tolerance, the molded body is rejected, which leads to a yield decrease. The stamper 3 is required to be exchanged when characteristics of the molded body are over an acceptable limit.
However, it takes considerable time to exchange the stamper 3, lowering productivity of optical disks. The number of exchange times for the stamper 3 should thus be reduced. To cope with this, a surface 2a of the mold 2 is coated in some cases to restrict the wear at the contact part 8 between the stamper 3 and mold 2. TiN is primarily used for the coating. While a wear resistance of the surface 2a of the mold 2 is improved in the presence of the coating layer, the TiN is adhered to the surface 2a of the mold 2 by plasma vapor deposition, in other words, an adhesive force of the TiN to the mold 2 is not strong. As such, if the coating layer is partly separated by the above-discussed friction force in the I direction, an abrasive wear acting on the stamper 3 by the separated TiN is stronger because the TiN has a hardness as large as approximately ten times the hardness of the stamper 3. The cut waste of the stamper 3 scraped off by the separated TiN produces the projecting parts 22 of the recording face 9a of the stamper 3, similar to the previously-mentioned case. With the use of the coating layer, although the stamper 3 undergoes less adhesive wear, the separated TiN brings about projecting parts 22 on the recording face 9a, making it difficult to secure mechanical characteristics of the optical disk as the molded body.
For solving the above disadvantages, fats and oils such as wax or the like, or lubricant oil are sometimes applied to the surface 2a of the mold 2. However, the fats and oils such as wax or the like lack stability under high temperatures for a long time. Moreover, the fats and oils emit moisture contained therein when heated, thus adversely influencing the stamper 3 or mold 2. To uniformly apply the fats and oils to the surface 2a of the mold 2 is a difficult task, and reproducibility is poor. In addition, foreign matter such as dust, etc. readily to adhere to the fats and oils to thereby decrease the quality of the molded body. Metallic components contained in a mineral oil which is a kind of the lubricant oil react to and affect the stamper 3 and mold 2.